1. Technical Field.
This invention relates generally to radio frequency (RF) integrated circuits, and more particularly, to RF integrated circuits utilizing a Doherty amplifier.
2. Related Art.
Wireless transmission devices such as cellular phone handsets, cellular base stations, and radio or TV transmitters employ a transmit signal to convey information within a communication system. To boost the transmit signal in these wireless devices, the signal may be passed through one or more power amplifiers. The ratio of transmitted power delivered by the power amplifier to the power consumed by the power amplifier is defined as efficiency. It is desirable that the power amplifier operates as efficiently as possible to minimize heat generated within the power amplifier. Moreover, where a battery powers the wireless device, it is desirable that the power amplifier operates as efficiently as possible to minimize the current drain on the battery.
Efficiency is only one consideration for a power amplifier. To guarantee system performance under worst-case conditions, a wireless device is designed to transmit at a specified Maximum Output Power. However, where the signal at a receiving device is of adequate strength or where a modulation scheme used in the communications system produces instantaneous variations (peaks and nulls) in the amplitude of the transmitted signal (e.g. analog AM modulation and numerous digital modulation schemes), a wireless device may typically operate at a power level below the Maximum Output Power.
With respect to Maximum Output Power, power amplifiers may be categorized as linear or non-linear. The bias or direct current (DC) operating point of an amplifier determines if an amplifier is linear or nonlinear. Amplifiers are further categorized in classes. In order of ascending efficiency and descending linearity, the classes include Class A, Class AB, Class B, and Class C.
An output signal of a linear amplifier is nearly identical to its input signal except that it is amplified by the gain of the amplifier. For example, if the input signal is increased, the output signal will increase by the same amount. An output signal of a non-linear amplifier may be different from its input signal. For example, an increase in the input signal of a non-linear amplifier may not result in an increase in the output signal if the amplifier is “saturated.” The same is true if the amplifier is operating at its Maximum Output Power. In general, non-linear amplifiers operating at or near the Maximum Output Power are more efficient than linear power amplifiers.
A conventional power amplifier typically operates with a fixed load line (or load impedance). The load line is the impedance (ideally a resistance) that is seen by a transistor or vacuum tube that may make up an amplifier stage. For a given power supply or battery voltage, the load line determines how much power an amplifier can deliver and is chosen as a design parameter to simultaneously achieve the desired Maximum Output Power and peak efficiency.
At power levels below the Maximum Output Power, there is more supply voltage or current available than what is needed for a desired transmit power level. Thus, the conditions for efficiency are no longer optimal at power levels below the Maximum Output Power. Moreover, the efficiency is lower than the peak efficiency value attained at Maximum Output Power.
To improve the efficiency over a conventional power amplifier, circuit designers may employ a Doherty Power Amplifier. The improvement resides in that a load line for a Doherty Power Amplifier may adjust dynamically to maintain high efficiency over a range of output (transmit) powers. Typically efficiency may be optimized over a range of power of 4 to 1, (6 dB).
The Doherty amplifier is named after its inventor, W. H. Doherty, who was responsible for early successful linear amplifier designs in the 1930s. The Doherty amplifier is a well-known linear radio-frequency power amplifier that is divided into two sections, section no. 1 (the “carrier amplifier”) and section no. 2 (the “peaking amplifier”). Section no. 1 typically is a Class B or Class AB type linear power amplifier and section no. 2 typically is a Class C type non-linear power amplifier.
It is a challenge to properly bias each of the two different types of amplifiers in a Doherty amplifier. Bias is required to ensure that the respective amplifiers only draw current and conduct at the appropriate load power signal levels. This challenge is made more difficult by manufacturing process variations and environmental variations.
For example, the operating point of one Doherty amplifier to the next may vary due to slight changes in the manufacturing of each device having the Doherty amplifier. Moreover, a wireless device employing a Doherty amplifier may be required to operate over a wide range of temperatures. Further, the power supply voltage to the Doherty amplifier may experience fluctuations as the battery repeatedly is cycled through charge and discharge periods.
Thus, for a Doherty amplifier, there is a need to generate appropriate bias signals, a need for a circuit that is capable of generating these bias signals, and a need for a circuit that is capable of dynamically compensating for process and environmental variations while generating these control signals. In particular, there is a need to develop such a circuit that may be implemented in a high volume/low cost Doherty Power Amplifier suitable for wireless devices such as cellular phones and other personal communications devices.